1. Field of the Invention
The present invention relates to an abnormality detection apparatus for a tool for detecting abnormal conditions represented by a condition which a wear and abrasion degree of the tool to be used for machining of a machine tool is over limit, and also relates to a numerical control apparatus provided with such abnormality detection apparatus for a tool.
2. Related Art Statement
Conventionally, as a time taken for machining has elapsed, a cutting resistance of a nose of a tool to be used for machining increases as a wear and abrasion thereof progresses, finally the tool is broken. As the wear and abrasion of the tool progresses, precision in machining is deteriorated, whereby a specified precision in machining necessary to a work piece cannot be kept any more.
On the other hand, an unattended machining and a prolonged automatic machining have been thought to be matters of course. Therefore, an apparatus for automatically detecting abnormal conditions such as a wear and abrasion limit of the tool and the like has been required, as a result a wear and abrasion of a tool monitoring apparatus disclosed in JP 11-58113 or the like has been developed.
Such wear and abrasion of a tool monitoring apparatus detects a torque load acting upon a spindle motor from a load current thereof, on the other hand, detecting a thrust load acting upon a feed shaft motor from a load current thereof, comparing the detected maximum torque load with a warning torque load set as a threshold value for displaying both of them on display means, while comparing the detected maximum thrust load with a warming thrust load set as a threshold value for displaying both of them on the display means, noticing an operator that a wear and abrasion of the tool comes to a limit thereof.
However, such wear and abrasion of a tool monitoring apparatus described above has following problems. Since the torque load detected from the load current of above-mentioned spindle motor and the thrust load detected from the load current of the feed shaft motor change on the basis of a feed rate of the feed shaft, the torque load and the thrust load are comparatively kept low even though the wear and abrasion of the tool comes to the limit thereof in machining at a low feed rate, while the torque load and the thrust load are comparatively kept high even though the wear and abrasion of the tool is at an early stage thereof in machining at a high feed rate, therefore the warning torque load and the warning thrust load cannot be set uniformly. Consequently, it has been necessary to set an appropriate warning torque load and an appropriate warning thrust load respectively for each feed rate in above-mentioned wear and abrasion of a tool monitoring apparatus.
Lately, as a spindle drive system and a feed drive system of a machine tool have been improved in speed and acceleration thereof, an operation of the feed drive system is controlled by an adaptive control with using such improved machine tool aiming at a safety and highly effective high-speed cutting. The adaptive control monitors a cutting load acting upon the spindle drive system and the feed drive system, gradually increasing the feed rate of the feed drive system to make the cutting load up to a preliminarily set value so that the spindle drive system and the feed drive system can display optimal functions thereof. A change of the cutting load (the thrust load) in a drilling process is shown in FIG. 6 when the feed rate of the feed drive system is gradually increased by the adaptive control. With the use of a coated carbide solid drill with an oil hole (MDW085MHK available from Sumitomo Denko Co., Ltd.) having a diameter of 8.5 mm, a work piece of FC250 was drilled at a cutting speed of 150 m/min for formation of holes each having a depth of 25.5 mm.
In FIG. 6, a first peak indicates the cutting load at a feed rate (feed per revolution) of 0.2 mm/rev, a second peak indicates same at 0.25 mm/rev, a third peak indicates same at 0.3 mm/rev, a fourth peak indicates same at 0.35 mm/rev, and a fifth peak indicates same at 0.4 mm/rev. In addition in FIG. 6, a line A indicates the cutting load set as a target value of the adaptive control, and a line B indicates a lower limit of the cutting load set to detect abnormalities represented by a break of a tool or the like.
As shown in FIG. 6, the cutting load is 1000N when the feed rate is set at 0.2 mm/rev while it is 1800N at 0.4 mm/rev, which shows that the cutting load increases quantity thereof almost by 1.8 times when the feed rate is doubled. Therefore, if the cutting load is set to 1500N in machining at the feed rate of 0.2 mm/rev with using a tool having already reached a tool life thereof (a tool life judged by a wearing condition), machining at the feed rate of 0.4 mm/revwith the cutting load of 1800N is thought to be impossible wherein the tool life is judged upon above-mentioned 1500N as a threshold value. In other words, machining by the adaptive control is impossible. On the contrary, if the cutting load is set to 2500N in machining at the feed rate of 0.4 mm/rev with using the tool having already reached the tool life thereof, machining may be performed over the tool life at the feed rate of 0.2 mm/rev wherein the tool life is judged upon above-mentioned 2500N as a threshold value, whereby the tool can be broken. Thus, it can be understood that the tool life cannot be judged by making use of a uniformly set threshold value under the adaptive control.
The present invention has as its principal object the provision of an abnormality detection apparatus for a tool which can judge abnormal conditions of the tool by using a uniformly set threshold value in relation to a cutting load for machining wherein a feed rate changes every moment especially for machining under an adaptive control, and also the provision of a numerical control apparatus provided with such abnormality detection apparatus for a tool.
The present invention according to a first embodiment for attaining above-mentioned object relates to an abnormality detection apparatus for a tool used for machining provided to be installed to a spindle of a machine tool, comprising cutting load detecting means to detect a cutting load on the basis of an electric current for driving that is output to a feed drive system and/or an electric current for driving that is output to a spindle drive system of the machine tool, feed load estimating means to estimate a feed load on the basis of a feed rate of the feed drive system wherein the feed load is a load component depending on the feed rate, tool load detecting means to pick out a load component depending on the tool by removing the feed load data estimated by the feed load estimating means from the cutting load data detected by the cutting load detecting means, and abnormality judging means to judge whether the tool is in an abnormal condition or not on the basis of the load data picked out by the tool load detecting means.
According to the present invention, a cutting load (a total cutting load) is detected on the basis of the electric current for driving that is output to the feed drive system and/or the electric current for driving that is output to the spindle drive system of the machine tool, on the other hand, the feed load as the load component which depends on the feed rate of the feed drive system is estimated. Then, the load component (a tool load) which depends on the tool is picked out by reducing the estimated feed load from the detected total cutting load. The tool load picked out is compared with a preliminarily set threshold value for judgement whether the tool is in the abnormal condition or not.
Thus, according to the present invention, since a tool life is judged upon the tool load picked out after the feed load or the load component depending on the feed rate of the feed drive system has been removed from the total cutting load and the load component depending on the tool has been completely picked out, the tool life can be correctly judged with using the uniformly set threshold value even in machining wherein the feed rate of the feed drive system changes every moment. Therefore, troublesome such that a setting of the threshold value for judgement must be changed whenever the feed rate changes as in a conventional apparatus can be removed.
Data as a basis of judgement of above-mentioned tool life is not limited by the tool load according to the first embodiment, but can be used for computing a wear and abrasion degree of the tool whereupon the tool life can be judged as in the invention according to a second embodiment. Here, the present invention according to the second embodiment comprises wear and abrasion degree computing means for computing the wear and abrasion degree of the tool on the basis of the load data picked out by the tool load detecting means in addition to the means of the first embodiment, wherein the abnormality judging means judge whether the tool is in the abnormal condition or not upon the wear and abrasion degree computed by the wear and abrasion degree computing means. Furthermore, the wear and abrasion degree of the tool can be indicated by making use of the ratio of a tool load of a new tool to that of a used tool having been used in machining for a specified time period. Although the tool load according to a first embodiment includes a component in relation to the feed rate, the wear and abrasion degree of the tool indicated by above-mentioned ratio of the tool load of the new tool to that of the used tool having been used in machining for a specified time period doesn""t include such a component in relation to the feed rate, therefore the tool life can be securely judged.
The present invention according to the first and second embodiments can judge the abnormalities of the tool by making use of the uniformly set threshold value without being affected by the feed rate of the feed drive system, whereby great effects can be expected under the adaptive control wherein the feed rate of the feed drive system is changed according to the cutting load especially in the numerical control apparatus according to a third embodiment.
The present invention according to the third embodiment is the numerical control apparatus which numerically controls the feed drive system and the spindle drive system of the machine tool, provided with the abnormality detection apparatus for a tool according to the first and second embodiments, comprising adaptive controlling means for accelerating or decelerating the feed rate of the feed drive system on the basis of the cutting load detected by the cutting load detecting means.
Tools whose abnormalities can be detected according to the present invention include a tool installed to a spindle of a machining center provided to be used for machining, a rotary tool of a lathe for a complex machining and almost all tools used in the machine tool.